With the increasing concern for the world environment protection, the harmful components in the exhaust gas of automobiles will be controlled stringently. In response to such controls, the quality of the fuel is required to be higher and higher. Thus, many countries have brought forward strict limitations on the quality of automobile gasoline, for example: oxygen content, vapour pressure, benzene content, total aromatic content, boiling point, olefin content, sulfur content etc. Thus, it is required by CAAA (USA) that, in nine most severely polluted states, by the year 2004, the sulfur content in RFG (Reformulated Gasoline) be less than 30 ppm, and the olefin content be less than 8.5%. The European Parliament also enacted an act which required that, by 2005, the sulfur content and olefin content in the gasoline be less than 30-50 ppm and 18% respectively. Accordingly, it is an important subject in the petroleum refining industry how to further reduce sulfur and olefin contents in gasoline.
By comparing the current Standards for Automobile Gasoline in China with Tier II quality standard of <<World-wide Fuel Charter>>, it can be concluded that the main problem about the automobile gasoline quality in China is the high contents of sulfur and olefins. High levels of sulfur and olefins in the automobile gasoline is mainly due to the high proportion of the fluidized catalytic cracking (FCC) gasoline in the gasoline pool. In China, the FCC gasoline is a main blending component, accounting for more than 80% in the gasoline blending pool. The FCC gasoline contains high levels of sulfur and olefins, especially when the feed of the FCC becomes heavier; thus, it is hard to obtain automobile gasoline with an olefin content less than 20%. At present, gasoline products from many refineries in China barely meet the current quality standard for automotive gasoline. Under the circumstances, a major approach to control the contents of sulfur and olefins in automobile gasoline is to reduce the contents of sulfur and olefins in FCC gasoline.
It is true that the conventional hydrogenation processes can be used to substantially reduce the contents of both sulfur and olefins in FCC gasoline. The hydrogenation process, however, results in olefins, high octane-number components, being saturated. Consequently, this process leads to heavy octane number loss, especially in the case of a gasoline with a relatively high content of olefins and a relatively low content of aromatics. FIG. 1 schematically shows a relationship between the octane number loss of a typical FCC gasoline, with high olefin content and low aromatics content, and the saturation degree of the olefins therein as the gasoline undergoes conventional hydrorefining. From the figure, it can be seen that, with the olefins being saturated higher, The loss of octane number of the gasoline product becomes higher when the olefin content is reduced to 19.3% by volume from 49.3% by volume, RON loss of the gasoline product is 12.3 units; when the olefins are completely saturated, RON loss of the gasoline product is 23.5 units. Obviously, it is more difficult to recover the octane number of the FCC gasoline, with high olefin content and low aromatics content, when processed by conventional hydrorefining process. In view of this, there is a need to develop a process for treating FCC gasoline to reduce its contents of sulfur and olefins with minimum octane loss.
U.S. Pat. No. 5,411,658 discloses a gasoline hydrorefining process, comprising employing a traditional hydrorefining catalyst to hydrorefine FCC gasoline, and then employing a β-zeolite catalyst to restore the octane number of the hydrorefined gasoline. The process was designed to treat a feedstock with a high final boiling point. Nevertheless, the process employs a high hydrorefining temperature and thus makes a large amount of aromatics saturated. As a result, RON octane number decreases substantially in the final product and is hard to restore.
U.S. Pat. No. 5,599,439 discloses a process for upgrading gasoline and reformate. The process comprises, in a first stage, hydrorefining the feed to remove the impurities of sulfur, nitrogen etc and saturate olefins. Then the effluent from the first stage is subjected to intermediate separating step; the gas with hydrogen sulfide, nitrogen, etc removed is directly recycled to the first stage, and the intermediate product oil enters the second step where it undergoes octane number restoring process in a fluidized bed reactor with no fresh hydrogen introduced therein. This process introduces a separator between the first and second stages, thus increasing the capital investment. In the meantime, the process employs a low operation pressure which adversely affects the long-term operation of the catalyst.
U.S. Pat. No. 5,391,288 discloses a gasoline upgrading process. In the process, the feed comprises the mixture of FCC oil and benzene-rich fraction oil isolated from the effluents of the reforming process. The process includes two reaction steps. In the first step, the feed material undergoes hydrorefining to reduce the contents of the impurities, such as sulfur, nitrogen etc, and at the same time to saturate olefins through hydrogenation. In the second step, the effluent from the first step is subjected to octane number restoring treatment in the presence of an acid-functional catalyst, mainly undergoing alkane cracking, and alkylation and transalkylation of the aromatics. The process adopts a relatively low space velocity in the hydrorefining step and employs a large amount of catalyst. Moreover, it employs a feed with benzene and thus produces a product containing relatively high content of aromatics.
U.S. Pat. No. 5,399,258 also discloses a process for upgrading gasoline. The process includes two steps. In the first step, the feedstock is subjected to hydrogenation to remove sulfur and nitrogen and saturate the olefins. The product from the first step directly enters the second step where it undergoes octane number restoring. The first step is operated at a high temperature, similar to that of the second step. Nevertheless, owing to the high reaction temperature in the first step, in the final product of the process, a large amount of mercaptan sulfur remains.
With gasoline feedstocks with a relatively low final boiling point, a relatively high level of olefins and a relatively low content of aromatics, the above-mentioned processes, when applied to reduce sulfur and olefin contents in gasoline feedstock, would lead to significant octane loss.
Therefore, there exists a need for a process for hydrotreating gasolines, especially those having a relatively low final boiling point, and a relatively high content of olefins and a relatively low content of aromatics, to deeply reduce the levels of sulfur and olefins, with minimum octane loss.